Low density dielectric coating for electrode in electron tube

ABSTRACT

MAGNESIUM CARBONATE IS MIXED WITH AN ADDITIVE IN A LIQUID CARRIER AND DEPOSITED IN A THIN FILM ON A SUBSTRATE. SUITABLE ADDITIVS ARE CERTAIN HIGH VAPOR PRESSURE COMPOUNDS WHICH DECOMPOSE INTO VOLATILE PRODUCTS AT RELATIVELY LOW TEMPERATURES WITHOUT PASSING THROUGH A LIQUID PHASE. AFTER DEPOSITION THE FILM IS ALLOWED TO DRY ON THE SUBSTRATE AND THEN THE COATED SUBSTRATE IS BAKE AT A SUFFICIENT TEMPERTURE TO PURGE THE ADDITIVE AND CONVERT THE MAGNESIUM CARBONATE TO MAGNESIUM OXIDE. THE VOIDS LEFT BY VAPORIZATION OF THE ADDITIVE RESULT IN A LOW DENSITY POROUS LAYER OF MAGNESIUM OXIDE ON THE SUBSTRATE WHICH HAS DESIRABLE MECHANICAL AND ELECTRICAL PROPERTIES. SUITABLE ADDITIVES ARE AZODICARBONAMIDE, AMMONIUM CARBAMATE AND AMMONIUM TETRAFLUOROBORATE.

United States Patent Office US. Cl. 117-222 9 Claims ABSTRACT OF THE DISCLOSURE Magnesium carbonate is mixed with an additive in a liquid carrier and deposited in a thin film on a substrate. Suitable additives are certain high vapor pressure compounds which decompose into volatile products at relatively low temperatures without passing through a liquid phase. After deposition the film is allowed to dry on the substrate and then the coated substrate is baked at a sutficient temperature to purge the additive and convert the magnesium carbonate to magnesium oxide. The voids left by vaporization of the additive result in a low density porous layer of magnesium oxide on the substrate which has desirable mechanical and electrical properties. Suitable additives are azodicarbonamide, ammonium carbamate and ammonium tetrafluoroborate.

BACKGROUND OF THE INVENTION This invention relates to an improved dielectric-coated electrode for an electron tube and has particular reference to an electrode for use as a storage target in an electrical image storage tube or as an electron emitter of general application.

A conventional storage target comprises a conductive element such as a screen or plate coated with a layer of dielectric material. The target is normally mounted inside an evacuated envelope such as a glass tube with the dielectric layer facing a plurality of electron guns also mounted inside the envelope. An image in the form of a charge or voltage pattern is placed (written) on the target, read and removed (erased) from the target through operation of these guns. The operation of such tubes is well understood by persons skilled in the art.

For certain purposes the target should be capable of accommodating relatively high writing speeds, referring to the linear travel speed of a beam of writing electrons on the dielectric surface. This is especially important where it is desired to produce an image of an event of momentary duration. Also, for certain purposes the target should be capable of storing an image for a considerable period of time.

High writing rate storage cathode ray tubes are desirable for single event or single transient wave form observation. This is an oscillographic application. Data transmission as used in computers, for instance, is employing higher clock, or bit rates (higher frequencies). In order to display information at these higher rates, an intermediate memory system has heretofore been required. This memory accepts data at a high rate and then re-transmits data at a lower rate acceptable by conventional display storage cathode ray tubes. A storage cathode ray tube with a high writing rate could eliminate this expensive intermediate memory.

High speed oscillography requires a camera when single events or low repetition rate events are to be observed, such as high voltage surges and chemical reactions. A cathode ray tube oscilloscope combined with a high speed camera and fast film can record single events at a writing rate in excess of 2000 10 cm./sec. In comparison, a single event written at x10 cm./ sec. is virtually invis- Patented Aug. 29, 1972 ible to a person viewing a cathode ray tube screen and must be photographed.

Storage cathode ray tubes are capable of holding a written trace for subsequent viewing. But the fastest conventional storage cathode ray tubes can record and store a single event only on the order of 5 megahertz (approximately l6 10 cm./sec. writing speed). It is desired to accomplish a much higher writing speed in order to obviate the necessity for auxiliary memory systems or photography for displaying high speed oscillographic events.

Objects of the invention are, therefore, to provide an improved storage target and electron emitter and methods of making same, to provide a storage which accommodates substantially higher writing speeds than have been usable hertofore with known targets, to provide such a target which is capable of retaining an image over a considerable period of time without appreciable image deterioration, to provide a porous low density dielectric target or emitter coating which has improved mechanical strength, to provide such a coating which has low die-, electric capacitance, low charge leakage or migration, high resistance to writing beam damage and high charge gain, to provide a coating which will go into conduction at high electric field strength without damage, to provide a simple process for applying such a coating, to provide a process in which it is easy to control the physical parameters and to provide a process which can be used on sub stratevthat require relatively low tempearture processing.

SUMMARY OF THE INVENTION The present dielectric coating is characterized by its porosity, low density and high mechanical strength and coherence as Well as improved electrical properties as mentioned above and described hereinafter in detail.

These desirable qualities are obtained by a simple two step process. In the first step a coating is deposited on the substrate comprising a mixture of a magnesium compound with an additive in a suitable liquid carrier. An additive is selected which is volatile or which decomposes into volatile components without passing through a liquid phase, at a temperature below that of the decomposition temperature of the magnesium compound. The coating is allowed to dry and is then baked at a temperature which purges the additive and converts the magnesium compound to magnesium oxide.

This results in a porous dielectric film of low density which has adequate strength for electron targets and electron emitters. Such coating on the target of a storage ray tube allows writing speeds above megahertz (approximately 314x10 cm./ sec.) to be directly viewed and then recorded by photography, if desired, without special photographic techniques. This writing speed is to be compared with the 5 megahertz writing speed of the fastest previously known storage cathode ray tubes mentioned above.

The invention will be better understood and additional objects and advantages will become apparent from the following detailed description of certain preferred embodiments. Various changes may be made in the composition and in the details of the process and all such modifications within the scope of the appended claims are included in the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Magnesium carbonate (MgCO decomposes in air to magnesium oxide (MgO) at temperatures above 350 C. If magnesium carbonate is deposited on a substrate in the form of a thin layer, for example less than .005 inch thick, and baked at a temperature above 350 C., a semiporous structure with an approximate density of 50% of the bulk density of the material results. This film structure has adequate strength for electron targets and electron emitters but the efficiency is reduced becmuse of the relatively high density.

According to the present invention this high density is reduced by the addition of a compound that is volatile or decomposes into volatile products at a temperature below the decomposition temperature of the magnesium carbonate. If the additive volatilizes or decomposes at a tem-' perature above 350 C., it is possible that the additive rnight react or combine with the magnesium carbonate to produce some other compound than the desired magnesium oxide.

Examples of suitable additives for the present purpose are azodicarbonamide (H NOcNzNCONfi which decomposes at 225 C., ammonium carbamate which decomposes at 60 C., and ammonium tetrafiuoborate (N-HgBF which sublimes at room temperature.

The magnesium carbonate and additive are mixed in a liquid carrier, such as water, alcohol or acetone, in approximately the proportions of 1 part magnesium carbonate to 10 parts additive to 100 parts liquid, on a volume basis.

The resulting mixture is agitated, poured over the substrate and allowed to settle, the final thickness of film being controlled by the amount of the mixture used. Other coating techniques well-known in the art of phosphor deposition may also be used such as spraying, electrophoresis, etc.

After deposition, the film and substrate are allowed to dry and are then baked at a temperature above 350 C. in air for approximately an hour, first causing the additive to decompose or vaporize and then causing the magnesium carbonate to decompose to magnesium oxide. The additive is thus purged without chemical reaction with the magnesium carbonate, leaving microscopic voids having an aggregate volume equal to ten times the volume of magnesium carbonate.

This process fabricates a porous layer of magnesium oxide with a desired controlled density below ten percent of the bulk density of the oxide. Since the reaction that results in magnesium oxide takes place at this low density, it is believed that intermolecular bonds are formed that add strength to the film structure. Although the nature of the crystalline transformation is not fully understood, it is found in practice that improved mechanical strength results, which makes the film highly suitable for the intended purposes. Without the added strength characteristic such a low density coating would be too fragile for the present purposes.

The above-described process has the advantages of being extremely simple and requiring relatively little equipment. Also, it is easy to control the physical parameters of the process and the process is well suited for substrates that require relatively low temperature processing.

Other suitable magnesium compounds are magnesium bromide (MgBr which converts to magnesium oxide at 700 (3., magnesium perchloarte (Mg(ClO which decomposes to magnesium oxide at 251 C., and magnesium sulfide (MgS) which converts to magnesium oxide at 400 C.

Such a coating has a number of particular advantages for electron targets. It has low charge leakage or migration. Since the conduction paths through a porous insulator are relatively long and discontinuous, larger voltage gradients are possible. This means that higher voltages can be maintained both through the dielectric thickness and across the dielectric surface.

The coating has high resistance to writing beam damage. Since this porous dielectric is a high temperature refractory material and has a small physical cross section, it is relatively resistant to damage by the high energy writing beam which impinges upon it. The coating has a high voltage breakdown. Many conventional solid dielectric films exhibit irreversible damage owing to dielectric breakdown. The present low density dielectric seems to go into conduction at high electric fields without damage.

The present coating has an improved charge gain. Since a. voltage gradient can exist within the structure of the 5 coating and the solid elements of this structure are very thin, the opportunity for secondary electron emission, surface ionization, photo ionization, photo con-duction or other conduction mechanisms is increased. As a consequence, more charge can be displaced Within the structure than is deposited by the writing beam.

Having now described my invention and in what manner the same may be used, what I claim as new and desire to protect by Letters Patent is:

1. A method of forming a porous, low-density dielectric coating on a substrate comprising the steps of preparing a dispersed mixture of particles of a magnesium compound which converts upon heating in air to magnesium oxide and particles of an additive which decomposes upon heating into volatile components without passing through a liquid phase,

applying a thin coating of the dispersed mixture to the substrate, and

applying sufiicient heat to the substrate to volatize the particles of additive and subsequently to convert the magnesium compound to magnesium oxide.

2. A method of forming a porous, low-density dielectric coating on a substrate comprising the steps of preparing a dispersed mixture of particles of a magnesium compound which converts upon heating in air to magnesium oxide and particles of an additive which decomposes upon heating into volatile components without passing through a liquid phase in a volatile liquid carrier,

applying a thin coating of the dispersed mixture and liquid carrier to the substrate,

'volatizing the liquid carrier to form a dry coating of the dispersed mixture on the substrate, and

applying sufficient heat to the substrate to volatize the particles of additive and subsequently to convert the magnesium compound to magnesium oxide.

3. A process as described in claim 2 including a mixture of one part of magnesium compound and between ten and one hundred parts of additive.

4. A process as described in claim 2 wherein the magnesium compound is selected from the group of magnesium carbonate, magnesium bromide, magnesium perchlorate and magnesium sulfide.

5. A process as described in claim 4 wherein the additive is selected from the group of azodicarbonamide, ammonium carbamate, and ammonium tetrafluoborate.

6. A method of forming a porous, low-density dielectric coating of magnesium oxide on a substrate comprising the steps of preparing a dispersed mixture of particles of magnesium carbonate and particles of azodicarbonamide in the proportions of one part magnesium carbonate to ten parts azodicarbonamide in one hundred parts alcohol,

spraying a thin layer of the dispersed mixture upon the substrate,

drying the substrate,

baking the substrate at a temperature above 350 C.

in air for approximately one hour to volatize the amdicarbonamide and subsequently to decompose the magnesium carbonate to magnesium oxide.

7. A substrate having a porous, low-density dielectric coating formed thereon by the process of claim 6.

7 8. A method of forming a porous low-density dielectric coating on a substrate comprising preparing a mixture of particles of a magnesium compound and of a nonmagnesium-containing additive, said compound being one which will convert above a certain temperature to magnesium oxide, and said.

additive being one which volatizes without liquifying at a temperature below said certain temperature,

applying said mixture to said substrate, and

heating the substrate and the blend thereon first to volatize said additive, and then to convert said compound to magnesium oxide.

9. A method of forming a porous low-density dielectric coating on a substrate comprising preparing a mixture of particles of a magnesium compound and of a nonmagnesium-containing additive, said compound being one which will convert above a certain temperature to magnesium oxide, and said additive being one which volatizes without liquifying at a temperature below said certain temperature,

blending said mixture with a liquid carrier in which the particles of said compound and of said additive are insoluble, said carrier being one which volatizes at a temperature below the volatizing temperature of said additive,

applying the blend to said substrate, and

heating the substrate and the blend thereon first to volatize said carrier, next to volatize said additive, and then to convert said compound to magnesium oxide.

References Cited UNITED STATES PATENTS 2,873,218 2/1959 Dobischek et a1 117222 2,950,994 8/1960 Firth 117-222 3,410,932 11/1968 Woodson et al. 117232 X ALFRED L. LEAVITT, Primary Examiner 

